This invention relates to a speed governor for a pneumatic power tool of the type including a housing with a pressure air inlet passage, a rotating output spindle, an air turbine including a turbine wheel drivingly coupled to the output spindle and one or more air nozzles in the housing for directing motive pressure air onto the turbine wheel, wherein the speed governor comprises a valve element arranged to control the pressure air flow through the inlet passage and a valve element activating piston means which is exposed to a speed responsive control pressure.
The problem solved by the invention is to accomplish a simple yet reliable speed governor for a fast rotating pneumatically powered turbine. Practical problems relating mainly to dynamic balancing and frictional wear in connection with fast rotating parts make it very difficult to use mechanical fly weight activated governors of any of the types commonly used on vane motor driven pneumatic tools. Examples of prior art speed governors of this type are illustrated in for instance U.S. Pat. Nos. 2,674,229 and 3,421,414.
Prior art speed governors for air turbine driven tools are described in U.S. Pat. Nos. 3,708,240 and 4,776,752. These governors are intended specifically for small size reaction type turbines, where the motive air flow passes through the rotor and nozzles mounted on the rotor. This type of governor requires a good rotating seal between the turbine wheel and the housing to seal off the motive air inlet passage, and there are some problems to obtain a good dynamic rotor balance when using these known turbine and known governor designs.
In another prior art patent, GB 727,649, there is described an action type air turbine provided with an overspeed control device which instead of mechanical speed responsive means associated with the turbine wheel utilizes a control pressure activated inlet valve to reduce the inlet flow at a certain speed level. In this known device, however, the control pressure is derived from a second medium transported by a pump coupled to the turbine, which arrangement reduces drastically the possible applications of the turbine control means to pump drives.
The object of the present invention is to accomplish an improved speed governor for an air turbine driven power tool wherein the above mentioned problems related to prior art technique are avoided. In particular, the invention intends to accomplish an air turbine speed governor having an air inlet flow controlling valve which is activated by a control pressure derived from the motive air flow without incorporating any corotating mechanical means which would jeopardize the dynamic balance of the turbine wheel.
Other characteristic features and advantages will appear from the claims.
A preferred embodiment of the invention is hereinafter described in detail with reference to the accompanying drawings.